When considering the ventilation system for a large building, the requirement to control airflow in the event of fire is self-evident. The basic solution to that problem was the fire damper, exemplified in U.S. Pat. No. Re. 30,204, originally issue to Root in 1976. That disclosure is typical: A damper assembly, consisting of an array of damper blades movable between open and closed positions, is mounted in a ventilation duct. A tensioning means (a spring or the like) is connected to the damper blade array to hold the array in a closed position. Then the blade array is rotated to an open position and held there by a fusible link, a metallic element that would separate at a rated temperature, allowing the tensioning means to close the damper. Thus, the presence of fire near the damper would automatically result in shutting off the airflow in that portion of the ventilation system.
It was found, however, that a disproportionate amount of damage and injury in fire situations was caused by smoke, not by the flames themselves. Because fire dampers automatically shut off airflow, firefighters could not selectively use the ventilation system to exhaust smoke from portions of the building affected by the fire. The solution to that problem was the combination smoke/fire damper, as seen in U.S. Pat. No. 4,332,272, issued to Becelaere in 1984. There, the fusible link is dispensed with altogether. The blade array is still controlled by a tensioning spring, but a damper actuator operates against the spring torque to open the blade array. A temperature sensing device can automatically disconnect the damper actuator and close the blade array, but that device can be overridden, allowing firefighters open the damper on command.
This approach suffers from two drawbacks. First, the damper actuator must always operate against the spring torque. That design wastes energy by requiring the motor to overcome the spring tension even when no fire is present. Second, this system fails to provide a fail-safe closure method. Because an operator can always override the temperature sensor, the damper may well remain open not only in the presence of smoke (which may be desirable) but also in the presence of flames (which is never desirable).
The art has therefore only come halfway to meeting the challenge of controlling both fire and smoke. The present invention solves that problem by offering a system that allows full control of the damper array up to the point of a flame condition, without imposing a spring load on the damper actuator, while also providing the foolproof operation of a fusible link to insure shutoff at a rated fire temperature.